The present invention relates to gas turbines having a closed-circuit thermal medium cooling system for cooling the nozzle stages and particularly relates to a pyrometer mount for a closed-circuit steam-cooled gas turbine for determining the surface temperature of gas turbine components in the hot gas path.
In a multi-stage advanced gas turbine, the nozzle stages are cooled by a thermal medium, preferably cooling steam. The steam flows into a plenum in the outer band of the nozzle stage for impingement cooling of the outer band. The steam then flows through the vanes of the nozzle stages for impingement cooling the vanes and into a plenum in the inner band for cooling the inner band. The spent cooling steam returns from the inner band through cavities in the vanes and through the outer band. It will be appreciated that the nozzle stages lie in the hot gas path of the turbine. It has been found necessary to monitor the surface temperature of gas turbine components during operational use and particularly to monitor the operational temperature of the first-stage buckets which, of course, rotate in the hot gas path.
To accomplish this, a pyrometer is employed which has a line of sight through the steam-cooled outer band of a nozzle stage, for example, the second stage, downstream of the buckets whose operational temperature is to be measured, e.g., the first-stage buckets. As will be appreciated, the outer band and cover contain cooling steam which would normally interrupt any line of sight through the nozzle stage. That is, the pyrometer must be able to see through the cover and nozzle band along the line of sight without allowing steam to leak into the components of the gas turbine or into the hot gas path. Consequently, the present invention addresses the requirement for a pyrometer""s line of sight to pass through a steam-cooled nozzle without steam leakage.
In accordance with a preferred embodiment of the present invention, a pyrometer mount is provided enabling the pyrometer""s line of sight to pass through the nozzle""s outer cover and outer band while providing a joint between the mount and the nozzle stage without steam leakage. It will be appreciated that the nozzle stage is formed of an array of nozzle segments circumferentially arranged about the rotor axis. In the present invention, the outer cover and band of a selected nozzle segment are provided with a pair of openings in registration with one another and angled in a forward and circumferential direction. A pyrometer boss is disposed in the openings and extends between the outer cover and the outer band, terminating along the radial inner surface of the outer band in the hot gas flow path. The radially inner portion of the pyrometer boss is electron beam welded to the outer band. Particularly, the circumferential angle of the axis of the apertures and the boss received through the apertures permits the electron beam to weld about the margin of the boss and outer band from one side of the nozzle segment. By using an electron beam welding technique, reduced distortion and a better quality weld at that location is provided. The radial outer end of the pyrometer boss is preferably TIG-welded to the outer cover. By welding about each of the radially inner and outer ends of the boss, leakage-free joints are provided between the boss and the outer band and cover.
Upon installation of the boss in the nozzle stage segment, the boss is machined to provide a through aperture having an axis generally corresponding to the axis of the registering holes through the outer band and outer cover. Also, a seat is provided in the aperture of the boss to receive a tube coupling the boss to a pyrometer mounted on the turbine frame. Thus, a line of sight facilitating temperature readings of the first-stage bucket is established through the outer band and outer cover of a steam-cooled nozzle stage.
In a preferred embodiment of the present invention, there is provided a pyrometer mount and nozzle stage assembly for a closed-circuit thermal medium cooled gas turbine, comprising a nozzle stage segment having inner and outer bands with at least one nozzle vane extending therebetween, the bands and the vane adapted to lie in a hot gas path of the turbine, a cover spaced from the outer band and lying on a side thereof remote from the vane, the cover and the outer band defining a plenum for receiving the thermal medium, the outer band and the cover having openings therethrough in registration with one another and a pyrometer boss having an aperture therethrough disposed in the registering openings, the boss being welded to both the outer band and the cover to seal the cooling medium within the plenum without cooling medium leakage about the boss.